Submarine DC cables are widely used for long-distance power transmission. If the power transmission line is forced to be suspended due to a fault, it will not only affect the power supply, but also cause a lot of economic losses. The sea situation is complicated, the sea area is large, and the construction cost is high. Therefore, it is very important to determine the fault type and locate it accurately. Starting from the study of the most common open circuit and short circuit faults of cables, the existing fault detection method for submarine DC cables, such as the rectangular wave used in the low-voltage pulse method, is easy to form a test blind zone, waveform distortion and other problems, and the actual operation effect is poor. In this paper, the spread spectrum time domain reflectometry (SSTDR) method for fault detection and location of cross-linked polyethylene (XLPE) submarine DC cable is proposed. The incident signal is a high frequency test signal obtained by combining $m$ sequence modulated by Binary Phase Shift Keying (BPSK) and sine wave, which effectively avoids signal aliasing, has high positioning accuracy and solves the shortage of low voltage pulse method. The simulation technology is used to simulate the submarine DC cable with different fault types and different fault locations, which provides theoretical support for practical application.